WARNING: This document has been retracted by the author(s). Implementation of the protocol described herein is not recommended. Developers desiring similar functionality are advised to implement the protocol that supersedes this one
(XEP-0060).

The Jabber Event Notification Service (ENS) acts as a dispatcher and may be used by applications as a central point of collection for certain types of events that are of interest to them. Examples of events include:

User has logged in

A new message has arrived

User's avatar has changed

The coffee machine is empty

In Jabber, the role of the ENS has traditionally been filled by overloading the <presence/> packet type. However, this method was never designed to be used as a general publish-and-subscribe mechanism, and so has the following problems:

Dispatching of <presence/> packets is performed by the JSM (Jabber Session Manager), and so is not easily usable by components and other entities that don't connect via a client manager (c2s, CCM).

An entity cannot subscribe to the presence of a specific resource of another entity, only to any presence from that entity. This lack of granularity makes its difficult to use <presence/> in situations where large chunks of data must be dispatched to subscribers (eg avatars).

The protocol consists of two parts - the subscriber-to-ENS protocol, and the publisher-to-ENS protocol. Since there is no direct interaction between a publisher and a subscriber, it makes sense to seperate the two parts of the protocol.

The protocol operates in the 'http://xml.cataclysm.cx/jabber/ens/' namespace.

A reference implementation was formerly available at http://cataclysm.cx/jabber/ens.html.

An event can be defined as a change to the value of one or more properties of a resource.

In the ENS, an event type is referred to by a JID (Jabber IDentifier), including the JID resource. For example, consider a hypothetical publisher that is linked to an IMAP message store. It might notify the ENS of the fact the a message has arrived, deleted, or filed, using the following JIDs:

rob@imap.cataclysm.cx/NewMessage

joe@imap.cataclysm.cx/DeletedMessage

jim@imap.cataclysm.cx/FiledMessage

Alternatively, an end-user client that wanted to notify the ENS when its avatar changes might do so using a JID like "rob@cataclysm.cx/avatar"

A subscriber is a Jabber entity that receives notifications about events. Usually, a subscriber will be an end-user client, but it may be any Jabber entity.

As the name suggests, a subscriber can subscribe and unsubscribe to various events via the ENS. When it subscribes, the publisher responsible for the event it is subscribing to will be asked by the ENS to authorise the subscription request. To facilitate this, the subscriber may provide an XML fragment containing information that the publisher can use to authorise it. The use of this fragment is application specific.

Once subscribed to an event, the subscriber will receive any notifications that the publisher sends about that event.

A publisher is the Jabber entity responsible for actually sending event notifications to the ENS. A notification contains the event type JID of the event that occured, and an optional "payload" XML fragment, that is passed untouched by the ENS to the subscriber. The contents of this payload is application-specific and can be used to provide detailed information about the event to the subscriber. For example, in the case of the NewMessage event above, the payload might contain the contents of the To:, From: and Subject: headers of the message.

Additionally, the publisher is responsible for deciding who may subscribe to events it publishes. When the ENS receives a subscription request, it will ask the publisher to decide whether or not the subscriber may subscribe to a particular event. This authorisation request may also contain an XML fragment from the subscriber containing information that may be used for authorisation.

If it wishes, the subscriber may request a "reliable" subscription. This option guarantees that the subscriber will receive all notifications about this event (as far as the Jabber architecture guarantees delivery). This changes the semantics of the subscriber publish protocol - see section 3.6 for more details.

The actual error fragment in the packet is a direct copy of the one returned by the publisher when it fails the authorisation request from the ENS. If the publisher does not provide one, error code 503 (Service Unavailable) will be returned instead. If the publisher does not respond to the authorisation request (after an implementation-specific timeout), error code (Remote Server Timeout) will be returned.

The ENS, as described, works well. However, no provisions are made for a subscriber to find out which ENS a publisher is publishing to. It does a subscriber no good to subscribe to an event on the wrong ENS - the subscription would succeed (assuming the publisher allows it), but no notifications would ever be received.

There are several potential solutions, each with their problems:

Leave it to the subscriber to find the appropriate ENS outside of the ENS framework itself. This might be via a browse to the publisher, or maybe just entering it into their client's configuration. Obviously, this is very application-specific.

Force the publisher to publish to multiple ENSs, as necessary. This would require additions to the protocol (to tell the publisher of new ENSs), and would require the publisher to maintain a list of JIDs it needs to publish to. This solution is pointless - if the publisher is going to publish to multiple JIDs and maintain its own list, it might as well publish direct to subscribers.

Have some sort of ENS-to-ENS protocol, and have ENSs proxy publishes for other ENSs. This does not fix the problem, it just moves it away from the subscriber and into the ENS. An ENS will still need to find out which ENS the publisher is publishing to.

Integrate ENS into the session manager. This leaves us with a glorified presence system, and makes the ENS basically unusable by non-session-manager-based server components.

Currently, if a subscriber obtains a reliable subscription, and then disappears from the network (as an end-user client might), the ENS will continue to send notifications to it (ignoring errors) until it unsubscribes. If the subscriber never comes back, then ENS will send notifications forever (in theory).

At what point should even <reliable/> have its limits? Should we unsubscribe a reliable subscriber who bounces (ie. the Jabber server failed to deliver) more than, say, 10 publishes? Or maybe they should be unsubscribed if they do not respond (as in section 3.6) to anything for, say, 10 minutes?

<reliable/> is an interesting idea, but it may be that it is too problematic for its own good.

The topic of reliable subscriptions raises the question as to how long a subscription lasts. Should a subscription last forever (like <presence/> does), even across restarts of the server?

If end-user clients are to be subscribers, then under this scheme the ENS would have to subscribe to presence, so as to know when the client has disconnected. Since presence is a function of the session manager, this could have the effect of making the ENS less generic that we may like.

As I see it, basic presence will always be a function of the session manager, for the following reasons:

No ENS discovery problems

Subscriptions are maintained across sessions

In very widespread use (ie. everywhere)

I think the places where the ENS can boom will be in applications like avatars, and genuine event-based systems (like the kind described above - "you have new mail", "someone has scheduled a meeting with you in your online calendar", etc).

Appendix B: Author Information

Robert Norris

Appendix C: Legal Notices

Copyright

Permissions

Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.

Disclaimer of Warranty

## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ##

Limitation of Liability

In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.

IPR Conformance

This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA).

Appendix D: Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.

Appendix E: Discussion Venue

The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.

Appendix F: Requirements Conformance

The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".